The present invention relates to a system for controlling an electromagnetic clutch for an automatic transmission.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A No. 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes such as a starting mode, reverse excitation mode, drag mode, and two modes of lock-up engagement which are an accelerator pedal releasing condition and depression condition. One of the modes is selected in accordance with a position of a selector lever and driving conditions to control the electromagnetic clutch.
In the system, as shown in FIG. 8, relationships between engine speed and vehicle speed are illustrated. At the start of the vehicle, the transmission ratio is set at a maximum value. When the accelerator pedal of the vehicle is depressed and engine speed becomes higher than a set value, the electromagnetic clutch is engaged to start the vehicle. The vehicle speed and engine speed increase along the set maximum transmission ratio represented by (line L,Ne/V). When the engine speed and vehicle speed reach set values at point A under a driving condition, the transmission ratio starts to change (it upshifts) at point A of FIG. 8. At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced along horizontal line m (representing engine speed as constant) and finally reaches a minimum transmission ratio (line H). When the accelerator pedal is released, the engine speed and vehicle speed reduce along the line H. When both speeds reach a point B, the transmission ratio begins to increase. Thus, the transmission ratio is increased (downshifted) along a line M and reaches the maximum transmission ratio (line L). However under certain driving conditions the engine speed becomes Neu, as shown by a line M' in FIG. 8, namely, the engine speed becomes higher than the predetermined engine speed, which corresponds to the minimum changing line M of the transmission ratio, in a low vehicle speed range under V.sub.H when a choke valve is closed. In such a state, the vehicle is accelerated because of the high engine speed regardless of the intention of the driver. In order to prevent the acceleration of the vehicle, the driver depresses a brake pedal to brake the vehicle. However, since the electromagnetic clutch is locked at a vehicle speed higher than a predetermined speed V.sub.L (FIG. 8), a large force must be applied to the brake pedal.